Technical Field
The present disclosure relates to an optical element and an image apparatus, and particularly relates to a prism group and a projection apparatus.
Description of Related Art
Generally, a projection apparatus includes an illumination system, a light valve, and a projection lens. The illumination system is adapted for providing illumination beam to illuminate the light valve. The light valve then converts the illumination beam into an image beam, and the image beam is projected by the projection lens on a screen to generate an image on screen. As the projection apparatus projects a completely black image, the light valve (e.g. a digital micro-mirror device, hereinafter “DMD”) is unable to terminate all the lights to send to the projection lens and the screen. A basic brightness remains on the screen even with a completely black image. When the basic brightness is too high, the black image becomes not black enough. Hence, the contrast ratio of the image projected by the projection apparatus decreases.